System and method of generating equation-level diagnostic error messages for use in circuit simulation

ABSTRACT

A mechanism for providing equation-level diagnostic error messages for system models undergoing circuit simulations is discussed. The components in a model of a system being simulated are converted into multiple numerical equations where each equation corresponds to a component in the system being simulated or a topology equation for the system model. Each numerical equation is numerically analyzed in order to identify illegal configurations in the system. Upon detection of an error, an error message listing the components associated with the illegal configuration is generated for the user.

FIELD OF THE INVENTION

The illustrative embodiment of the present invention relates generallyto circuit simulation and more particularly to a mechanism forgenerating equation-level diagnostic error messages based on a numericalanalysis of equations associated with model components in a system beingsimulated.

BACKGROUND

Circuit simulators attempt to diagnose illegal configurations in systemmodels with a network topology that are being simulated and then presentthe information about the illegal configuration to a user. An example ofan illegal configuration in a system is a loop of independent voltagesources that violates Kirchhoff's voltage law (the sum of voltagesaround the loop must be 0; which is not the case when the loop hasmultiple independent voltage sources). Ideally, an error message willidentify all of the components involved in the illegal configuration fora user. Conventional mechanisms for accomplishing the diagnosis ofillegal configurations include performing structural analysis ofequations associated with the components in the system being simulatedand techniques that require the categorization of the components intofive separate classes prior to analysis.

Unfortunately, the conventional mechanisms for diagnosing illegalconfigurations in systems being simulated suffer from a number ofdrawbacks, especially in the area of multi-domain circuit simulation.Certain systems are not particularly amenable to having their componentscategorized into five classes (voltage sources, capacitors, resistors,inductors and current sources) prior to analysis, and for these systems,categorization is either impossible or forces the system designer/userperforming simulation to make awkward categorization choices. Theinability to properly categorize the system components leads at best toa significantly less accurate diagnosis of the system. The otherconventional mechanism that is frequently used, structural analysis,simply does not properly diagnose all types of illegal configurationswith which the user may be concerned.

BRIEF SUMMARY

The illustrative embodiment of the present invention providesequation-level diagnostic error messages for system models undergoingcircuit simulation. Circuit simulation in the present invention may beperformed on system models that include a network topology. The systemmodels may represent different types of domains such as electrical,hydraulic and mechanical and may include components from more than onetype of domain in a single model (a multi-domain model). The componentsin a model of a system being simulated are converted into multiplenumerical equations where each equation corresponds to a component inthe system being simulated or a topology equation for the system model.Each numerical equation is numerically analyzed in order to identifyillegal configurations in the system. Upon detection of an error, anerror message listing the components associated with the illegalconfiguration is generated for the user. The present invention does notrequire pre-categorization of the model components and successfullydetects illegal configurations not identifiable through conventionalstructural analysis techniques.

In one aspect of the present invention, a method for identifyingequation-level errors in a system model used in circuit simulationincludes the step of providing a representation of the structure of asystem model to be simulated, the system model including multiple systemmodel components. The method converts the representation of thestructure of the system to multiple numerical equations with eachnumerical equation corresponding to a system model component or atopology equation for the system model. The method additionallyidentifies at least one illegal configuration in the system model byperforming a numerical analysis of at least one of the numericalequations.

In another aspect of the present invention a system for identifyingequation-level errors in a system model used in a circuit simulationincludes a representation of the structure of a system model to besimulated. The system model includes multiple system model components.The system model also includes multiple numerical equations with eachnumerical equation corresponding to a system model component or atopology equation for the system model. Additionally, the systemincludes an analysis facility for identifying an illegal configurationin the system model that performs a numerical analysis of the numericalequations.

In one aspect of the present invention, a distributed system foridentifying equation-level errors in a system model used in circuitsimulation includes a first computing device accessed by a user. Thefirst computing device communicates over a network with a secondcomputing device. The system also includes the second computing devicewhich supports a circuit simulation environment. The circuit simulationenvironment includes a representation of the structure of a system modelto be simulated and an analysis facility. The representation of thestructure of the system includes representations of multiple systemmodel components that are converted into a plurality of numericalequations. Each numerical equation corresponds to a representation of asystem model component in the representation or to a topology equation.The analysis facility identifies an illegal configuration in the systemmodel by performing a numerical analysis of the numerical equations. Thesystem additionally includes a display device in communication with thefirst computing device that is viewable by the user. The display devicedisplays either an error message generated in the circuit simulationenvironment that identifies at least one component involved in theillegal configuration or it displays a graphical representation of thesystem model that includes at least one indicator associated with asystem model component that is identified as involved in the illegalconfiguration.

In an aspect of the present invention, a method for identifyingequation-level errors in a system model used in circuit simulationincludes the step of providing a representation of the structure of asystem model to be simulated, the system model including multiple systemmodel components. The method converts the representation of thestructure of the system model to multiple numerical equations with eachnumerical equation corresponding to a system model component or atopology equation for the system model. The method additionallyidentifies at least one illegal configuration in the system model, byperforming a symbolic analysis of at least one of the numericalequations. An error message is generated by the method that identifiesat least one of the system model components involved in the illegalconfiguration.

In another aspect of the present invention, a method for identifyingequation-level errors in a system model used in circuit simulationincludes the step of providing a representation of the structure of asystem model to be simulated. The system model includes multiple systemmodel components. The method also converts the representation of thestructure of the system model to multiple numerical equations that eachcorrespond to an equation associated with the system model.Additionally, the method identifies at least one illegal configurationin the system model by performing a numerical analysis of at least oneof the numerical equations.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the appended claims.The advantages of the invention described above, as well as furtheradvantages of the invention, may be better understood by reference tothe following description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 depicts an environment suitable for practicing the illustrativeembodiment of the present invention;

FIG. 2 depicts an alternate distributed environment suitable forpracticing the illustrative embodiment of the present invention;

FIG. 3A is a screenshot of a block diagram of a system with an illegalconfiguration detectable by the present invention and the associatederror message;

FIG. 3B is a screenshot of a block diagram of a multi-domain system withan illegal configuration detectable by the present invention and theassociated error message;

FIG. 3C is a screenshot of a block diagram of a system in which theillegal configuration causes a step size violation noted in anassociated error message;

FIG. 4 is a flowchart of a sequence of steps followed by theillustrative embodiment of the present invention to identify illegalsystem configurations by using a numerical analysis; and

FIG. 5 is a flowchart of a sequence of steps followed by theillustrative embodiment of the present invention to associate andutilize replacement components for model components that are identifiedas being part of an illegal configuration.

DETAILED DESCRIPTION

Multi-domain systems that have components from different domains such aselectrical, hydraulic and mechanical domains have proven difficult toanalyze for illegal circuit configurations using conventionaltechniques. The illustrative embodiment of the present inventionprovides a mechanism for translating component descriptions written indomain-specific languages into corresponding numerical equations. Theequations are numerically analyzed in order to identify illegal circuitconfigurations. Components involved in the illegal configuration arethen identified for the user via error messages listing the componentsor by providing indicators in a graphical representation of the modelbeing simulated.

FIG. 1 depicts an environment suitable for practicing the illustrativeembodiment of the present invention. A computing device 4 that isaccessible by a user 2 communicates with a display 20. The computingdevice 4 may be a PC, workstation, server, laptop, mainframe, PDA orother computing device equipped with a processor and capable of hostingthe circuit simulation environment 6. The computing device 4 may be avirtualized device. The computing device 2 may be equipped with a singleprocessor, a multi-core processor, or multiple processors. By way ofexample, FIG. 1 shows a processor 3 with multiple cores 5 and 7. Theprocessor may be a 32 bit processor, 64 bit processor or some other typeof processor capable of executing the instructions required for thepresent invention.

The computing device 4 hosts a circuit simulation environment that isused to simulate the execution of a system model 8. An exemplary circuitsimulation environment 6 is SIMDRIVELINE or SIMHYDRAULICS working inconjunction with MATLAB and SIMULINK, all from The MathWorks, Inc. ofNatick, Mass. The components of the system model 8 are represented in arepresentation 10 such as a graph that holds information about thestructure of the system being modeled. The components represented in thegraph or other representation 10 are converted from the high-leveldomain-specific language of the system model 8 to numerical equations12. Each numerical equation 12 is associated with a particular componentof the system model 8 that is represented in the graph 10.

An analysis facility 14 is utilized by the circuit simulationenvironment 6 to numerically analyze the numerical equations 14. Anumber of different types of numerical analysis may be performedincluding techniques based on sparse QR and reduced-row-echelon form.

The present invention requires a technique for factoring an n by nmatrix A into A=E*B, where E is of full column rank and B is instaircase upper-triangular form. While E must have full column rank itdoes not have to be explicitly computed. An example of staircaseupper-triangular form is given in the table below.

X ? ? ? ? ? ? 0 X ? ? ? ? ? 0 0 0 X ? ? ? 0 0 0 0 0 0 X

In the table, ‘X’ represents a definitely nonzero entry, ‘0’ representsa zero entry, and ‘?’ indicates that the value of the entry may be zeroor nonzero. The term “staircase form” indicates that each column'shighest-row-index nonzero entry has a row index at most 1 larger thanthe row index of the previous column's highest-row-index nonzero entry,except in the case of the first column, which must be either all ‘0’values or have exactly one nonzero entry (which must be at row index 1).As discussed herein the staircase matrix is required to have at leastone nonzero entry in each row. The columns in which thehighest-row-index nonzero entry has a row index equal to 1 larger thanthe maximum row index of all nonzero entries in all preceding columnsplay a special role and will be referred to as the step columns.

Two techniques which provide the desired form of analysis are sparse QRdecomposition and transformation to reduced-row echelon form. In sparseQR, a sparse A produces a sparse R such that A=Q*R (within some errortolerance) where Q is composed of orthogonal columns and R is staircaseupper-triangular. (Reduced QR, is used to produce an R without zerorows.) A transformation to reduced row-echelon form produces an ‘E’ suchthat A=E*B, where E is nonsingular, B is in staircase upper-triangularform and the step columns form the order-n identity matrix. (To get Eand B into the form needed from reduced-row echelon form, one needs toremove zero rows of B and remove the corresponding columns of E. E maythen be non-square but will still have full column rank.)Implementations of these analysis techniques are available in manycommercial and public-domain software packages.

The numerical analysis is performed in order to determine whether eachequation is independent of all of the previously processed equations.Any dependency detected indicates an illegal configuration in the systembeing modeled since the circuit equation systems are expected to besquare and have non-singular Jacobians. The indices of nonzerocoefficients in the dependency indicate to the analysis facility whichequations are involved.

For example, when solving a system of nonlinear equations, size m×n,whose Jacobian df/dx is provided by the simulation environment, denotethe system by f(x)=0, where x is a vector of n components (typically thesystem state vector) and f(x) is the vector of function residuals, oflength m. This form describes the DC (steady-state) problem as well asother problems of interest (such as the Initial Conditions solve inSIMHYDRAULICS). Define J(x_(—)0)=df/dx(x_(—)0) for an arbitrary x_(—)0.If there is a dependency in the rows of J, this indicates a possibledependency or inconsistency in the equations of f. Then defineA=J(x_(—)0)^T (transpose of J(x_(—)0)). The rows of J correspond toequations of f, which are either topology equations or componentequations. Each column of A is thus associated with either a topologyequation or a component equation and is used to generate diagnosticmessages relating back to the components. The generation of thediagnostic messages is discussed further below.

The association of the columns of A with a topology equation orcomponent equation is now described in greater detail. If A=E*B as inthe notation above B_i may be defined to be the step columns of B, B_dmay be defined to be the rest of the columns of B, and A_i and A_d maybe defined to be the corresponding columns of A. It should be noted thatB_i is invertible since B is in staircase form. Then E*B_d=A_d andE*B_i=A_i. Writing A_d=E*B_d=E*B_i*(inv(B_i)*B_d), we findA_d=A_i*(inv(B_i)*B_d). Columns of A_i are linearly independent since Eis of full column rank and B_i is nonsingular. It should also be notedthat the B_d may be empty (all the columns of B are independent)—in thiscase there is no illegal configuration to report. Thus the dependentcolumns of A (A_d), corresponding to dependent Jacobian rows, areexpressed in terms of the independent columns of A (A_i) through(inv(B_i)*B_d). Each column of (inv(B_i)*B_d) thus gives information ona dependency. The indices of nonzero entries of (inv(B_i)*B_d) are theindices of Jacobian rows involved in the dependency. In practice, asmall tolerance value should be selected and entries below thisthreshold in (inv(B_i)*B_d) changed to 0.

The technique described above can also be used to analyze systems forpossible problems in transient analysis. In particular, many ODEintegrators cannot handle systems of differential index >1. Consider thesystem (M_d 0;0 0)*x′=(f_(—)1(x);f_(—)2(x)), where M_d is diagonal ofsize n_d by n_d and the whole matrix on the left-hand-side is of sizen×n. This system is by definition of index >1 exactly when the matrixC=(M_d 0; df_(—)2/dx) is nonsingular for almost all x. Potentialproblems may thus be identified with the transient simulation by takingA=C′ and applying the technique described in the preceding paragraphs.

It should also be noted that the simulation environment may provideinformation on which equations are linear constant-coefficient (topologyequations, and some other types of equations as well, i.e. linearresistors). This can be used to refine the diagnostic tool to onlyreport dependencies involving only linear constant-coefficientequations, since these Jacobian rows will be dependent for all states x.Also, equations and variables may be scaled to improve the conditioningof A, thereby improving the reliability of the technique.

The analysis facility 14 generates an error message 26 for eachidentified illegal configuration. The error message may be displayed onthe display 20 for viewing by the user. Alternatively, the error messagemay be written to an error log file 16 or similar record. In oneimplementation, the error message identifies only the componentsinvolved in the illegal configuration in the error message.Alternatively, the identified illegal configuration may be presented tothe user via a graphical representation of the system model 22 that isdisplayed to the user. The analysis facility 14 may provide indicators24 in the graphical representation of the system model 22 that areassociated with the representations of the components involved in theidentified illegal configuration.

The circuit simulation environment 6 may also include one or morepossible replacement components 18 that may be used as substitutes forcomponents in an identified illegal configuration. The use ofreplacement components 18 is discussed in further detail below.

FIG. 2 depicts an alternative distributed environment suitable forpracticing the illustrative embodiment of the present invention. A user30 may utilize a first computing device 32 equipped with multipleprocessors 33 and 35 to communicate over a network 50 with a secondcomputing device 60. The second computing device 60 and other componentsin the distributed architecture may be virtual components. The network50 may be the Internet, a local area network (LAN), a wide area network(WAN), a wireless network, an intranet, an extranet or some other typeof network.

The second computing device 60 may include a virtual machine 61 thatsupports a circuit simulation environment 62. The user may create orretrieve a system model 34 at the first computing device 32 and entercommands to send the model over the network 50 to the second computingdevice 60 for simulation. A graph 64 or other representation is createdthat represents the structure of the system model 34. Each of thecomponents in the graph 64 is converted into a corresponding numericalequation 66. An analysis facility 68 numerically analyzes the numericalequations 66 in order to identify illegal configurations. The identifiedillegal configurations are conveyed to the user 30 via a graphicalrepresentation of the system model 42 that includes indicators thatindicate components involved in the illegal configuration or through anerror message listing the components involved. 44. Both the graphicalrepresentation of the system model 42 and the error message may betransmitted over the network 50 to the first computing device 32 so thatthey may be presented on the display 40 for the user to view.Alternatively, commands may be sent from the circuit simulationenvironment 62 on the second computing device 60 over the network 50that cause the first computing device 32 to generate the error message44 or graphical representation of the system model 42.

It will be appreciated by those skilled in the art that otherdistributed and parallel architectures enabling the execution of theinvention described herein are also within the scope of the invention.For example, the system model 34 which is depicted in FIG. 2 as beinglocated on the first computing device 32 may be located on the secondcomputing device 60 or on a third computing device accessible over thenetwork 50. Similarly, the circuit simulation environment 62 may belocated on the first computing device 32 that is accessible to the userwhile the system model 34 is retrieved from the second computing device60 that is accessed over the network 50. These and other configurationscapable of the functionality discussed herein are considered within thescope of the present invention.

As noted above, one of the benefits of the present invention is that itperforms a numerical analysis in order to identify illegal singularitiesin a system being simulated that would not be identifiable via astructural analysis. For example, consider the following set ofequations representing a system:v1=1v2=2v3=4v1−e1+e2=0v2−e2+e3=0v3+e1−e3=0This is an inconsistent system of equations since adding the first threeequations and subtracting the last three equations from the resultproduces 0=7. Conventional structural analysis techniques look for astructural singularity in the equations (i.e., a singularity whichexists independent of the values of nonzero coefficients in the linearsystem, but depends only on which coefficients are zero and nonzero.)Thus, the above system is structurally non-singular but is numericallysingular. The illegal system of equations is identifiable with thepresent invention but not with conventional techniques relying onstructural analysis. A simple example illustrating the importance ofusing a numerical technique is a voltage source connected in a loop witha resistor. The circuit is legal unless the resistance is zero. As aresult the legality of the circuit depends on the numerical value of theresistance (performing this analysis post-compile is useful since theresistance value could be defined by other blocks in the model ratherthan just as a hard-coded constant).

FIGS. 3A-3C provide illustrative examples of types of error detectionprovided by the present invention. FIG. 3A shows a system 70 whichincludes two velocity sources 72 and 74 which impose a relative velocityconstraint across the same two nodes. By compiling a graph of thestructure of the system into numerical equations which correspond to thesystem components, and then numerically processing the equations, thefact that both velocity sources are imposing a constraint on the sametwo nodes is identified. An error message 76 indicating the componentsinvolved in the illegal configuration is provided to the user.

FIG. 3B shows a multi-domain system with an illegal configurationdetectable by the present invention. The two domains in the picturedsystem 80 are mechanical-rotational 82 and mechanical-translational 84,and the “transformer” between them is a wheel and axle component 86. Theforce sensor 88 is required to have a linear velocity of 0 across thesensor (analogous to a current sensor in an electrical circuit, whichideally has no resistance, i.e. a 0 voltage across the sensor). In thedisplayed system 80 however, the angular velocity source 90 convertedthrough the wheel and axle 86 to a linear velocity, insists on aconstant 1 for the velocity across the same two nodes. The incorrectplacement of the force sensor is detected by the diagnostics of thepresent invention after compilation but before the time-domain(transient) solve starts. This type of multi-domain setting which may beanalyzed by the present invention includes types of components that arenot readily susceptible to categorization into the five classestypically used by conventional techniques to diagnose illegalconfigurations.

The illustrative embodiment of the present invention also allows moresubtle problems to be diagnosed. It one implementation, the numericalanalysis may reveal situations which are not illegal in the sense ofcreating an inconsistent set of algebraic equations, but may cause thesolvers to fail or take extremely small timesteps (a higher indexproblem). An example is a capacitor-voltage source loop. Anothervariation can identify situations in which it might be possible to startthe simulation from a specified initial state, but for which nosteady-state (DC) solution is possible (i.e., a current source chargingup a capacitor). FIG. 3C is a screenshot of a block diagram of a system100 in which an illegal configuration causes a stepsize violation. Whendifferential states are not truly independent, the solvers in thesimulator may have problems (as shown by the minimum stepsize violationin the error dialog 102). In this case the differential state v (fromM*dv/dt=f in the mass) is actually algebraically related to the input(from the flow rate source 104), through the hydro-mechanical converter106, which converts flow rate to velocity. The numerical analysis of thecomponent equations performed by the present invention reveal theillegal dependency and trigger the identification of the illegalconfiguration.

It should be noted that the present invention may be implemented todiagnose illegal configuration either at the time of initialization ofthe model or at the time of simulation. While slower than diagnosingproblems at the time of model initialization, the diagnosis of problemsat the time of simulation allows the diagnosis of problems caused bymodel components which dynamically receive their values at the time ofthe simulation.

FIG. 4 is a flowchart of a sequence of steps followed by theillustrative embodiment of the present invention to identify illegalsystem configurations by using a numerical analysis. The sequence beginswith the model of a system to be simulated being created or retrieved(step 120). A graph or other representation that represents thestructure of the system to be simulated is then created (step 122). Thegraph is compiled resulting in numerical equations that each correspondto a component in the system being modeled (step 124). A numericalanalysis is performed on each numerical equation (step 126).Representative numerical analysis techniques include techniques based onsparse QR and reduced-row echelon form. A determination is made as towhether the system contains an illegal configuration (step 127). If anillegal configuration is not detected, the system may be simulated (step128). If an identification of at least one illegal configuration is madebased on the numerical analysis an error message is generatedidentifying the system model components involved in the illegalconfiguration (step 130). The error message may list the componentsinvolved in the illegal configuration. Alternatively, the user may benotified of the illegal configuration by way of indicators applied to agraphical representation of the system.

One aspect of the illustrative embodiment of the present inventionallows for the replacement of problem components that have been found tocause simulation problems. “Safer” versions of components whichexperience has shown cause simulation problems are provided andassociated with the original components so that the diagnostic enginemay suggest replacements when illegal configurations are identified. Forexample, in an electrical system a small resistance in series withvoltage sources and in parallel with current sources would be added.

FIG. 5 is a flowchart of an exemplary sequence of steps followed by theillustrative embodiment of the present invention to associate andutilize replacement components for model components that are identifiedas being part of an illegal configuration. The sequence begins when areplacement component is associated with a model component of a systemto be simulated (step 140). Subsequently, when a numerical analysisidentifies an illegal configuration and the involved components (step142), a determination is made (step 143) as to whether a replacementcomponent is available for the component identified as involved in theillegal configuration. If no replacement component is available, anerror message is generated (step 144). If a replacement component isavailable, a message may be generated informing the user (step 146) andthe identified component may be replaced by the replacement component(step 148). Alternatively, the replacement may take place without noticeto the user or may wait for the user's approval. The simulation may thenbe run or re-run on the model that includes the replacement component(step 150).

In another aspect of the present invention, symbolic analysis may beused to diagnose and report errors in linear and non-linear systems. Asnoted above, structural analysis considers what variables participate inwhich equations. Structural analysis discards information that indicateshow the variables participate in each equation. In contrast, symbolicanalysis, like numerical analysis, utilizes the information thatindicates how the variables participate in each equation. The techniquesof the present invention that were described above may be extended touse symbolic analysis to diagnose and report errors for non-linearsystems.

For example, in one implementation of the present invention, eachequation in a non-linear system may be represented as an abstract syntaxtree (AST). With symbolic analysis, variables and equations may beeliminated by rearranging an equation such that one variable is writtenexplicitly as a function or new expression of other variables. The onevariable is eliminated from the system of equations by substituting thenew expression everywhere the one variable appears. Non-independent setsof equations arise when a variable substitution yields a trivial orinconsistent equation, e.g., 0=7. As in the case of the numeric analysistechniques discussed above, the discovery of a dependent equationtriggers the generation of an error message.

It will be appreciated that although the examples discussed above havediscussed the numerical equations as corresponding to either a systemmodel component or to a topology equation for the system model, thenumerical equations may also correspond to other types of equationsassociated with the system model. For example, the numerical equationsmay correspond to an equation that affects the environment in which themodel operates or may correspond to a dynamic equation associated withthe model that changes during simulation. In the case of errorsidentified after performing a numerical analysis of an equation notassociated with a particular model component or the topology of thesystem model, a general error message may be provided to the user.

Those skilled in the art will recognize that additional implementationsrelated to those described above are also possible within the scope ofthe present invention. For example, in one implementation, thenumerically-based technique of the present invention enables diagnosisinformation to be provided for problems during a simulation that occuronly in a certain range of operation of the component (e.g., if an idealdiode it acts like a 0-valued voltage source when the current ispositive and a 0-valued current source otherwise). Likewise, thenumerical analysis of the present invention may be utilized to finddependencies in presenting equations to explicit solvers when there arestate constraints. Similarly, the model components may be listed ashierarchical names to which the dependency-coefficient information fromthe numerical analysis can be mapped when providing an error message tothe user about a detected illegal configuration.

The present invention may be provided as one or more computer-readableprograms embodied on or in one or more mediums. The mediums may be afloppy disk, a hard disk, a compact disc, a digital versatile disc, aflash memory card, a PROM, a RAM, a ROM, or a magnetic tape. In general,the computer-readable programs may be implemented in any programminglanguage. Some examples of languages that can be used include MATLAB,FORTRAN, C, C++, C#, or JAVA. The software programs may be stored on orin one or more mediums as object code.

Since certain changes may be made without departing from the scope ofthe present invention, it is intended that all matter contained in theabove description or shown in the accompanying drawings be interpretedas illustrative and not in a literal sense. Practitioners of the artwill realize that the sequence of steps and architectures depicted inthe figures may be altered without departing from the scope of thepresent invention and that the illustrations contained herein aresingular examples of a multitude of possible depictions of the presentinvention.

1. A method for identifying equation-level errors in a system model usedin circuit simulation, comprising: providing a representation of thestructure of a system model to be simulated, the system model includinga plurality of system model components; converting the representation ofthe structure of the system model to a plurality of numerical equations,each numerical equation corresponding to a system model component or atopology equation for the system model; and identifying at least oneillegal configuration in the system model by performing a numericalanalysis of at least one of the numerical equations, wherein theidentified illegal configuration is an illegal component equation for acomponent in the system model.
 2. The method of claim 1 wherein thesystem model is a multi-domain system model.
 3. The method of claim 1,further comprising: generating an error message for a user identifyingat least one system model component involved in the illegalconfiguration.
 4. The method of claim 3 wherein only the at least onesystem model component involved in the illegal configuration isidentified.
 5. The method of claim 3 wherein the at least one systemmodel component is identified by a hierarchical component name.
 6. Themethod of claim 3, further comprising: providing a graphicalrepresentation of the system model being simulated; and providing anindicator in the graphical representation for each component involved inthe identified illegal configuration.
 7. The method of claim 1, furthercomprising: generating an error message for an error log that identifiesat least one component involved in the illegal configuration.
 8. Themethod of claim 1 wherein the illegal configuration is identified byidentifying a numerical equation corresponding to a system modelcomponent or a topology equation for the system model that is dependentupon a previously analyzed numerical equation associated with acomponent in the system model.
 9. The method of claim 8 wherein theindices of non-zero coefficients in a matrix in a description of thenumerical analysis are utilized to identify at least one equation and atleast one associated components involved in the illegal configuration.10. The method of claim 1 wherein the numerical analysis is one of thegroup of sparse QR or reduced-row-echelon form.
 11. The method of claim1 wherein the numerical analysis is performed during an initializationof the system model.
 12. The method of claim 1 wherein the numericalanalysis is performed during a simulation of the system model.
 13. Themethod of claim 1 wherein the identified illegal configuration is anillegal topology configuration in the system model.
 14. The method ofclaim 1 wherein the identified illegal configuration causes an illegalstep size simulation of the model.
 15. The method of claim 1, furthercomprising: associating a first replacement component with a firstcomponent in the system model; and replacing the first component in thesystem model with the first replacement component upon identifying anerror in the configuration of the system model that involves the firstcomponent.
 16. A system for identifying equation-level errors in asystem model used in a circuit simulation, comprising: a processor; arepresentation representing the structure of a system model to besimulated, the system including a plurality of system model components;a plurality of numerical equations, each numerical equationcorresponding to a system model component or a topology equation for thesystem model; and an analysis facility, utilizing the processor, foridentifying an illegal configuration in the system model, the analysisfacility performing a numerical analysis of at least one of thenumerical equations, wherein the identified illegal configuration is anillegal component equation for a component in the system model.
 17. Thesystem of claim 16, further comprising: a graphical representation ofthe system model, the graphical representation including at least oneindicator associated with a system model component involved in anidentified illegal configuration.
 18. The system of claim 16, furthercomprising: an error message identifying at least one component involvedin an identified illegal configuration.
 19. The system of claim 16wherein the analysis facility performs one of the group of a sparse QRor reduced-row-echelon form analysis.
 20. The system of claim 16 whereinthe system model is a multi-domain model.
 21. A medium holdingcomputer-executable instructions for identifying equation-level errorsin a system model used in a circuit simulation, the instructionscomprising: instructions for providing a representation representing thestructure of a system to be simulated, the system including a pluralityof system model components; instructions for converting therepresentation representing the structure of the system to a pluralityof numerical equations, each numerical equation corresponding to asystem model component or a topology equation for the system model; andinstructions for identifying an illegal configuration in the systemmodel by performing a numerical analysis of at least one of thenumerical equations, wherein the identified illegal configuration is anillegal component equation for a component in the system model.
 22. Themedium of claim 21 wherein the system model is a multi-domain systemmodel.
 23. The medium of claim 22, wherein the instructions furthercomprise: instructions for generating an error message for a useridentifying at least one system model component involved in the illegalconfiguration.
 24. The medium of claim 23 wherein the at least onesystem model component is identified by a hierarchical component name.25. The medium of claim 23, wherein the instructions further comprise:instructions for providing a graphical representation of the systemmodel being simulated; and instructions for providing an indicator inthe graphical representation for at least one system model componentinvolved in the identified illegal configuration.
 26. The medium ofclaim 21 wherein the numerical analysis is one of the group of sparse QRor reduced-row-echelon form.
 27. The medium of claim 21 wherein thenumerical analysis is performed during an initialization of the systemmodel.
 28. The medium of claim 21 wherein the numerical analysis isperformed during a simulation of the system model.
 29. The medium ofclaim 21, wherein the instructions further comprise: instructions forassociating a first replacement component with a first system modelcomponent in the system model; and instructions for replacing the firstcomponent in the system model with the first replacement component uponidentifying an error in the configuration of the system model thatinvolves the first component.
 30. A distributed system for identifyingequation-level errors in a system model used in circuit simulation,comprising: a first computing device accessed by a user, the firstcomputing device communicating over a network with a second computingdevice; the second computing device that supports a circuit simulationenvironment, the circuit simulation environment including arepresentation representing the structure of a system model to besimulated and an analysis facility, the representation includingrepresentations of a plurality of system model components that areconverted into a plurality of numerical equations, each numericalequation corresponding to a representation of a system model componentin the representation, the analysis facility identifying an illegalconfiguration in the system model by performing a numerical analysis ofat least one of the numerical equations, wherein the identified illegalconfiguration is an illegal component equation for a component in thesystem model; and a display device in communication with the firstcomputing device and viewable by the user, the display device displayingat least one of an error message generated in the circuit simulationenvironment that identifies at least one component involved in theillegal configuration or a graphical representation of the system modelthat includes at least one indicator associated with a system modelcomponent identified as involved in the identified illegalconfiguration.
 31. A method for identifying equation-level errors in asystem model used in circuit simulation, comprising: providing arepresentation of the structure of a system model to be simulated, thesystem model including a plurality of system model components;converting the representation of the structure of the system model to aplurality of numerical equations, each numerical equation correspondingto a system model component or a topology equation for the system model;identifying at least one illegal configuration in the system model byperforming a symbolic analysis of at least one of the numericalequations, wherein the identified illegal configuration is an illegalcomponent equation for a component in the system model; and generatingan error message identifying at least one system model componentinvolved in the illegal configuration.
 32. The method of claim 31wherein the system model is a multi-domain system model.
 33. The methodof claim 31 wherein the symbolic analysis is performed during aninitialization of the system model.
 34. The method of claim 31 whereinthe symbolic analysis is performed during a simulation of the systemmodel.
 35. The method of claim 31 wherein the identified illegalconfiguration is an illegal topology configuration in the system model.